The invention relates to a few methods of manufacturing a dispenser cathode, comprising barium and scandium compounds for dispensing barium to the emissive surface of a cathode body which consists substantially of a high melting-point metal or alloy.
There are, in addition to the oxide cathode, three other main types of dispenser cathodes, the L-cathode, the pressed cathode and the impregnated cathode. A survey of these three types of dispenser cathodes is described in Philips Technical Review, Volume 19, 1957/58, No. 6, pp. 177-208, which article is incorporated herein by reference. The characteristic feature of dispenser cathodes is that there is a functional separation between the electron-emissive surface and on the other hand a store of the emissive material which serves to produce a sufficiently low work function of said emissive surface. The emission of an L-cathode takes place from the surface of a porous metal body, the work function of which is reduced by adsorbed Ba and BaO. Behind the porous body the L-cathode has a storage space in which a mixture of tungsten powder and emissive material (for example barium calcium aluminate) is present. A pressed cathode and an impregnated cathode have a slightly different construction in which the storage space is absent and the emissive material is present in the pores of the porous metal body. A pressed cathode is formed by pressing a mixture of metal powder, for example tungsten and/or molybdenum powder and emissive material. An impregnated cathode is obtained by impregnating a pressed and sintered porous metal body with the emissive material.
A method similar to the one described in the opening paragraph is disclosed in U.S. Pat. No. 4,007,393. This Patent describes a porous metal body which is pressed from tungsten powder, sintered and which has a density of approximately 80% of the theoretical density. It is impregnated with a mixture which comprises 3% by weight of scandium oxide in addition to barium oxide, calcium oxide and aluminium oxide. The resulting cathode can provide a current with a current density of 5 A/cm.sup.2 at an operating temperature of 1000.degree. C. for approximately 3000 hours.
U.S. Pat. No. 3,358,178 describes a pressed dispenser cathode the cathode body of which is composed of tungsten powder and barium scandate (Ba.sub.3 Sc.sub.4 O.sub.9). The barium scandate forms 5 to 30% of the overall weight of the cathode body. With such a cathode a current density is obtained of 1.5 to 4 A/cm.sup.2 at 1000.degree. to 1100.degree. C. for a few thousand hours. During manufacture, such a cathode body must be sintered at approximately 1550.degree. C. for approximately 5 minutes after pressing. A higher sintering temperature would result in decomposition of the barium scandate. As a result of this comparatively low sintering temperature, the porosity of the sintered cathode body becomes so large, however, that the barium present easily diffuses towards the surface and then evaporates. Furthermore, the quantity of barium in the cathode is comparatively small as a result of which the life of the cathode is detrimentally influenced. This is the case certainly at operating temperatures above 985.degree. C.